The Five Fiddy

I find this picture unnerving.


     Easily the most frightening thing I have done as a pilot is contemplate purchasing a new engine for my plane.  I should say at the outset of this page that what follows will be profoundly bad reading for anyone who isn't deeply involved with aviation.  So if you aren't a pilot/owner you'd best go back to looking at the pictures.   But believe me, when you are flying along at night on one engine and it coughs slightly and shudders, that experience has a way of focusing your mind on this problem quite unlike anything else I have ever known.  Put differently; there isn't anything about the engine on my Viking I am not interested in knowing, because I'm sure I'd have a pretty bad day if it ever quit.

     With the above in mind it should be obvious buying a new engine is a decision you really want to get right.  But there are endless factors to weigh and options to pick from, and its hard to tease out the appropriate weight to give them.  For example: the TBO on my old engine, an IO-520K, was 1700 hours.  Numerous folks I respect say, "TBO is a meaningless number, the oil analysis is good, the compression is good, it has no leaks, just keep flying."  Mike Busch, one of my favorite writers on the topic, takes that position.  And it makes alot of sense.  But imagine me having this conversation with my wife, "Uh, honey, the people who made our engine rec om end it be taken out of service and overhauled at 1700 hours in service or 10 years.  We now have 1800 hours and 22 years.  I think I'm smarter than they are, so I'll just keep flying."

I showed my wife this picture and she said, "Its gold colored-- how appropriate."

     But what to do?  The IO-520 in my Viking was designed in the late 60's and built in the mid 80's.  It's a great engine and has given me great service.  But when I was at Miller's buying 12Y I had an opportunity to see a Viking being fitted with the newest of the big bore Continentals: the IO-550.  By all accounts folks this is a helluva engine.  It represents an additional 20 years of experience on Continental's part, and while it has virtually the same exterior dimensions as the 520 it has quite a few improvements.  It also has more horsepower.  How much is an interesting question.  What I gather is that the 520 produces 300 horsepower plus or minus a couple of percentage points and was certified to do this without the accessories. The 550 makes 300 horsepower only plus, no minus, a couple of percentage points, and it does this with the accessories.  The guys from Continental tell me alternators and fuel pumps and vacuum pumps can rob as much as 30 horsepower or more.  The conservative guess is that the 550 makes about 10% more horsepower than the 520.

    The buzz I'd heard about the 550 Viking was also very intriguing.  The sales material from Alexandria indicates the 550 Vikings are cruising around the 205 mph mark, and Dwayne Mitchell at Miller's Flying Service, now Rocket Aviation, who has done about a half a dozen 550 conversions, said it really climbed like all get out, and went like stink.  Well heck, who wouldn't like that? 

     So I started talking with Tom Witmer at Witmer's Aircraft Service in Pottstown, PA about the possibility of doing the conversion, and running the numbers to see what it would cost.  And boy, the numbers are breathtaking.  But I had a small advantage; my '85 Viking has the McCauley prop and Woodward governor which are compatible with the 550.  If I'd had to buy a new prop and governor that would have been the end of the analysis.  Too much money.  Both the prop and governor have to be slightly reworked to work on the 550, but they were going to be overhauled anyway at the engine change, so the incremental difference didn't seem like a big deal. (Ed. note: these small incremental differences add up.  Like I said, the numbers are breathtaking.)

Larry, one of Tom Witmer's guys, fitting a a new rear baffle.

     While working the problem of the prop I learned a couple of interesting things.  The maximum length for my McCauley prop when installed on a 520 is 78".  The minimum length for that same prop when installed on a 550 is 78".  So, first I needed to find out if my prop had been shortened somewhere in its life.  I flew to Lancaster, PA and the guys at Sennsenich came out and measured it for me.  They were happy to do it, as they are shop Witmer uses for prop work and would eventually do the overhaul.  Turns out I had enough blade length, but just barely.   So I've made a mental note to myself: DON'T DING THE PROP.  There is nothing extra to file away in the event I whack a big stone with the tip.

     In 2002 Bellanca altered type certificate A18CE for the 17-30A, my airplane, which now reads "Engine-- Continental IO-520-K Or Continental IO-550-F."  The "Or" is the key word there.  As I understand it, and more importantly, as my local FSDO interprets it, that means the 550F is a direct substitution for my old 520K.  No STC required, and no paperwork needed, other than Tom Witmer's endorsement in my log book.  Its worthy of mention that while Tom Witmer had installed many 550s in Bonanzas pursuant to various STCs, 12Y was the first Viking he converted.  The rub here is that I hate to be the first guy to do anything. 

     It is impossible to discuss the matter of the 550 engine conversion on a Viking without at least some mention of Rocket Aviation (formerly Miller Flying Service), and Dwayne Mitchell, the head maintenance guy there.  Plainview, Texas, where Rocket is located, is the Mecca of the Viking world, and Dwayne is widely respected as the last word on Viking maintenance issues.  I asked Dwayne for an estimate on doing the conversion, his number was roughly the same as Witmer's, and because Witmer's is about fifteen minutes away I decided to go with Tom.  I have spoken with several Viking owners who have had their conversion done in Plainview and are very happy, though I suspect there may be some small philosophical differences between the products offered by these two shops which might be said to reflect the slightly differing philosophical inclinations of Tom and Dwayne.  For example, Tom bought a full set of new engine baffles from the factory for my conversion.  My understanding is that Dwayne routinely reworks the old baffles.  When I put the Rocket estimate side by side with Witmer's estimate the bottom line numbers were nearly identical, but the labor and parts breakdown was different.  Rocket was heavier on labor and lighter on parts, while Witmer was the other way around.  When I asked Tom about this he just shrugged and said that his way all the part numbers on my converted plane will be identical to the factory 500 Viking.  The bottom line is that any differences between the product offered by these two shops is probably invisible to the average pilot.


     The 550F(17) comes equipped with an altitude compensating fuel pump.  When this was pointed out to me in my initial conversation with the engine sales guy I simply went along with him, faking like I knew what he was talking about, ashamed of my ignorance.  When I finally got around to asking someone about it they said, "Oh yeah, that's the auto lean feature of the 550."  I had no idea at all what that was about.  There is nothing about it in the Bellanca AFM, nor was Continental engine handbook very helpful.  All it says is, "These engines are equipped with altitude compensating fuel pumps which automatically provide the proper full rich mixture for any given altitude."  So after asking alot of questions here is what I learned: the altitude compensating fuel pump in my plane is a device that changes the mixture for you. Duh.

     To understand how it works you need a few examples.  First example, you take off with the mixture full forward and climb to ten thousand without touching a thing.  The altitude compensating fuel pump will lean the mixture as you climb, keeping you consistently rich as you go.  The mixture knob doesn't slide out by itself or anything, but the pump is (at least in theory) leaning the mixture to a consistent number of degrees rich of peak as you climb.  Example two:  You depart IFR and are cleared to 2000 expect 5000 in ten minutes.  So you climb to 2000 with the mixture knob all the way in.  At 2000 you decide to set up for 70% power and lean to 50 degrees rich of peak and spin out the mixture knob to accomplish that.  Shortly thereafter you are cleared to 5000, you maintain your 70% power setting by advancing the throttle as you go to keep the manifold pressure the same.  When you level out at 5000 you are still 50 degrees rich of peak.  Example three:  You are cruising along at 5000 at 70% power and 50 degrees rich of peak and are told to descend to 4000.  You maintain your 70% power setting as you descent by backing out the throttle.  When you level out at 4000 you are still 50 degrees rich of peak.

    So how, you ask, does this thing work?  To get that you need to understand the wave.  The wave is the leaning schedule the mechanical brain of the fuel pump follows as you climb.  As you can see from the picture below as you climb the fuel pressure changes.  This graph shows how the gallons per hour changes as you climb.  While Continental and Bellanca didn't say diddly about this in the operational info there is a forty page Service Bulletin from Continental on how to set up the fuel system so your fuel consumption falls between the two lines of the wave.  SB--  is some of the worst reading I have ever done, but I thought it was crucial to understand what its all about because I'd heard a number of scarey stories about the 550.  The worst was a guy with a Beech Barron who had a pair of them installed for some astronomical price.  He said emphatically the 550 was a pice of crap.  He'd had a terrible time with CHTs and the cylinders lost compression in 250 hours.  What I subsequently learned in conversation with the customer service guys at Continental is that many operators over the years have just swapped 550s for their old 520s and neglected to do the fuel pump set up procedure described in the Service Bulletin, assuming it was adjusted correctly at the factory.  If you fail to set up the pump properly for your individual installation the system may lean too much and you can cook your new $35,000 engine in a few hundred hours.


The set up procedure involves ground adjustments using a special calibrated fuel flow box, which Tom Witmer has, and a test flight to 14,000 feet checking fuel consumption along the way to make sure the pump is doing what its supposed to do.  The graph you see above is the result of our first test flight.  You can see my engine came from the factory out of spec.  Too rich at low altitudes and and too lean at high altitudes.  Service bulletin ____describes the procedure which gets this back into shape.


   The service bulletin is pretty emphatic that they want you flowing 24.6 GPH on take off at sea level on a standard day. That sounds easy enough-- just turn the screws until that's what you get, right?  Well not so fast.  I had my manifold pressure/fuel flow gauge re-silk screened and recalibrated as part of the this project.  There are some different colored arcs associated with 550 operations which had to be put on the face.  But when it came back I expected it would more or less agree with the very accurate fuel flow information on my EDM 700.  Wrong.  The freshly rebuilt gauge disagreed with the EDM by about 2 gallons per hour.  That's nearly a 15% error at typical cruise power settings.  My inclination was to believe the EDM, mostly because it just looks like it ought to be more accurate, it's digital and it reads out in tenths of a gallon.  Tom Witmer's view was that because the plane was certified with the steam gauge we had to go by that, despite the fact that it reads out in, at best, single digit increments.  The problem is that two gallons one way or the other is easily enough to put you outside the acceptable band shown on the wave chart.  Tom was anxious enough about this that he sent the gauge back to the overhauler to be checked and got their calibration sheet.  The gauge checked out perfect.  So who should we believe for the purpose of setting up the fuel flows, the ship's gauge or the EDM?   Tom and I basically agreed to split the difference.  As you can see from the chart below the ship's gauge tracks the upper limit and the EDM tracks the lower limit. Which one is right?  I don't really know.

     You might say, heck, who cares?  Well, Continental does.  Setting up the fuel system according to the Service Bulletin is one of the things you have to do to get their extended warranty.  The last thing I want is to make a warranty claim and have Continental thumb their nose at me because I didn't jump through all the right hoops.


   It became evident to me early on that the folks in Alexandria haven't focused much energy on the 550 conversion for the existing fleet.  There is no kit for sale where all of the necessary parts are gathered and the whole thing is tied together as a package.  They were perfectly happy to sell all the individual parts, but at the consumer end there was lots of head scratching and phone calling to try to figure things out.  This was particularly evident in the case of the Aircraft Flight Manual (AFM).  Alexandria sent the updated flight manual, but there were no power tables showing how to run the engine.  After a couple of months I was finally able to get a set of 550 Viking power tables from an owner with a 2000 Viking with the factory 550.  The first thing I learned was that I'd been running the plane all wrong.  For want of any better idea I'd been using the same power settings as I had on my old 520.  Turns out the 550 has about two extra inches of manifold pressure.  24"/2400 was 75% power on my old 520, with the 550 I get 75% at 22"/2400.  Glad I figured that one out.

      There are a number of stories circulating about power settings on the 550 Viking.  The one I like best is that the plane is so fast with the 550 that the factory didn't show all the available power settings on the published power tables, because if they had the plane would have cruised in the yellow arc and that is a certification no-no.  I like that story because it suggests the plane is dangerously fast with the new engine.  While it is true my plane is noticeably faster, the story is not entirely true.  Remember how I said the 550 has a couple extra inches of manifold pressure?  When I used the 75% cruise power setting (24" x 2400 ) applicable to my old 520 (because I didn't have the new power tables) I was probably making something over 80% with the 550.  Put another way, 24" and 2400 rpm is not an available cruise power setting with the 550 because its more than 75% power, and Continental doesn't want you to lean above 75% power, so it didn't make sense for Bellanca to put those numbers in the table.  But I didn't know any better, and found myself flying around in the middle of the yellow arc.  This was both worrisome and exhilarating at the same time.  Now that I'm using correct power settings I find I cruise just a hair outside the yellow arc.  I'm not fooling myself when I flight plan for 165-70 knots.   Nice.


      My engine is a factory reman, and because the core I turned in (a 520) wasn't the same type of engine as I was purchasing (a 550) I really had no idea what I was going to get back.  The crankshaft or the case, or any other part really, except for the camshaft, which Continental guarantees is new, could have thousands of hours on it.  As long as it passes their QC tests its supposed to be as good a new; that's the theory anyway.  So after I wrote the huge check for the engine I got wondering about this and called up customer service at Continental.  The guy at the help desk asked for my serial number and in a second he was able to report that my case, crank, cylinders, rods etc; virtually everything on my engine was new, except for the mags and the fuel system.  I feel like I lucked out.  The difference in price between a new engine and a remanned is around $8,000 and my engine is 98% brand new.


      Cooling issues are not uncommon with the 550. I routinely hear about this in Bonanzas and Barons which have been converted from the 520 to the 550. While the peak allowable CHT for the 550 is 460 f., 40 degrees higher than my old 520, I have no desire to see numbers like that.  I set the alarm threshold on my JPI at 400 f. and may occasionally see that on take off on a hot day. When the alarm does start to blink I bring the nose right down and the problem goes away immediately.      A number of folks I have spoken with who have the 550 conversion in their Viking have had difficulty with the #5 cylinder running hot. This is not surprising; #5 sits right behind the oil cooler. The answer has been to remove a portion of a baffle which sits between the oil cooler and they cylinder. The baffle was obviously placed there by Continental to keep the heat from the oil cooler away from the cylinder; it's a little counter intuitive but the removal of this baffler seems to solve the problem.  At the beginning, this wasn't an issue for me. Tom Witmer and his guys did an exquisite job sealing the baffles and getting the rubber gasket material around the cowl to fit very tight. They used silicone RTV, artfully placed, to seal up every seam and hole, and it has really paid off.  Initially my CCHTs were pretty close together and ran in the 350 f. range at an OAT of 70 degrees with cowl flaps in trail. Though my #5 cylinder is always the hottest.  Usually around 35 degrees hotter than the others.      After about a year of ownership I decided to revisit the number five cylinder.  It became something of a bother.  It would hit 400 f. (my JPI alarm threshold) on take off in warm weather and would prevent me from closing the cowl flaps when all the other cylinders were down in the 350s. While it was easy to keep everything under my self imposed limit of 380 f. it was annoying to be flying around with all of my cylinders in the 340s or lower and number 5 at 378f. I decided that I ought to do what a number of other 550 Viking owners have done and remove the baffle.     I asked Tom Wittmer to alter he baffle and he was initially skeptical, but I plotted out the colored graph the JPI software permits you to make showing engine temps and it was clear #5  was running noticeably hotter than the rest.  Tom is a by the book guy and was reluctant to change the factory recommended configuration.  After a little arm twisting by me Tom offered to split the difference; he said he’d remove half the baffle, and if that worked OK he’d consider taking off the rest.    The difference was noticeable on the first flight.  #5 was 15 degrees cooler and the spread between my CHTs was noticeably reduced, so at annual the rest of the baffle is going.  My hope is that with the baffle completely gone  #5 my average CHT will be lower and the temps grouped closer.

     A final note for ’79 and later Viking owners: closing the cowl flaps has a huge impact on the performance of the cabin heat.  When the cowl flaps are closed the air pressure at the pick ups at the front of the cowling for the cabin heat is much higher resulting in much better heat and defrost.


 At 74% power at 6000 feet at 41 degrees f. 12Y delivers a TAS of 200 mph even.  In that configuration I am burning a little more than 17 gph at around 70 degrees ROP.  I am just tickled about this. That is just outside the bottom of the yellow arc. What a way to travel.